build: detect macOS architecture for CMake

[dmorn]

Fixes #413.
Related to #329 and #377.

What changed

build.sh now sets CMAKE_OSX_ARCHITECTURES on macOS when the caller has not already provided it.

The default is derived from the running Erlang VM via erlang:system_info(system_architecture), with a uname -m fallback. This keeps the NIF architecture aligned with the VM architecture and avoids Rosetta shell/host mismatches.

The existing SDKROOT setup now also preserves a caller-provided SDKROOT instead of unconditionally overwriting it.

Why

msquic's OpenSSL/QuicTLS Darwin configure path expects CMAKE_OSX_ARCHITECTURES to be arm64 or x86_64. If CMake leaves it empty, the build can fall through to the generic OpenSSL config path (ERRORWTF darwin in older msquic), causing architecture mismatch and/or bad -isysroot compiler flags.

Verified

On Apple Silicon/macOS:

bash -n build.sh
rm -rf c_build priv _build msquic && mkdir -p priv && ./build.sh v2.5.5
~/.mix/elixir/1-19-otp-28/rebar3 compile

Confirmed generated cache/configure output includes:

CMAKE_OSX_ARCHITECTURES:STRING=arm64
CMAKE_OSX_SYSROOT:PATH=/Applications/Xcode.app/.../MacOSX26.4.sdk
-- Configuring OpenSSL: ARCHFLAGS="-arch arm64";...;darwin64-arm64-cc ...

[dmorn]

Kinda duplicate of #331